It is well known to convert an analog video signal, such as the video components of a television signal, from its conventional analog form into a digital signal. Such a digital signal has a large bandwidth, and in order to reduce this various video signal compression schemes have been proposed.
One such compression scheme is described in Dischert et al. U.S. Pat. No. 4,320,416 issued Mar. 16, 1982 and entitled "Technique for Optimally Encoding Digitally Encoded Video Signals". In this scheme, alternate 8-bit samples of the digital video signal are not transmitted, and instead of each such sample which is not transmitted two "filter control" or steering bits are transmitted. At a receiver, the steering bits are used to determine which of four possible averages of adjacent pairs of transmitted samples is used to reconstruct the non-transmitted sample. This scheme provides a compression ratio of 16:10 (two 8-bit samples are replaced by one 8-bit sample and two steering bits) or 1.6.
The bandwidth of proposed HDTV (high definition television) systems is much greater than that of conventional television signals, requiring very high bit rates for digital transmission of HDTV signals. An effective compression scheme, with a high compression ratio, for digital HDTV signals is therefore particularly desirable. However, it is important that any compression scheme which is used should not seriously detract from the relatively high definition of such signals, at least as far as the visual perception of the viewer of the eventually reproduced video signal is concerned.
For distribution of digital video signals, it is desirable that the signals be conveniently accommodated by existing and proposed communications systems. In particular, it is desirable that such signals be conveniently distributable via optical communications systems operating in accordance with the SONET (synchronous optical network) standard. In such systems, an STS-3c signal payload provides a bit rate of 149.76 Mb/s, and it is desirable for a digital HDTV signal to be accommodated within this bit rate. An STS-3c signal is an STS-3 level signal in which the STS (synchronous transport signal) envelope capacities of 3 STS-1s are concatenated and transported as a single entity.
An object of this invention, therefore, is to provide an improved method of compressing digital video signals.